Opsin synthesis was measured in miniature poodles affected with progressive rod-cone degeneration. Photoreceptors develop normally but begin to degenerate after the dog is fully grown. At all ages studied the rate of rod outer segment renewal was about half the normal value. Opsin synthesis, however, occurred at the normal rate until advanced stages of the disease when photoreceptor cell death was apparent. Thus the defect may involve photoreceptor membrane assembly rather than synthesis.